Improve Docs to Better Help Devs Bootstrap

[linuxwolf]

As a developer, I would like to know quickly how to get started helping out.

• [ ] /README -- link to ./docs/developers/install.md
• [ ] /docs/README.md -- link to ./developers/install.md
• [ ] Do something about /docs/user-guide.md?
• [ ] Link to built .xpi (once we have a .xpi to be installed)
• [ ] Remove scare strong from contributing.md
• [ ] Be explicit about prerequisites (correct node.js version, cloning the repo)
• [ ] Promote "run" or "watch" over "build" / "package"
• [ ] Hints about command-line args (e.g., "-f nightly")
• [ ] Clean up MkDocs (GH pages)